The ginger paradox (6) Reward

Edit: The capitalisation of Reward is sarcasm. End edit.

Corn oil is very Rewarding™ for rodents. These people are working hard at the mechanism:

Sham feeding corn oil increases accumbens dopamine in the rat

so let's not assume that Reward is some airy fairy concept, it's fully physiological. How Rewarding corn oil is in rodents is beautifully illustrated by the paper I  discussed recently in which mice, when given the choice, voluntarily consumed pure corn oil until it comprised roughly 84% of the total calories in their diet. Of pure fat.  Without training of any sort. This is the paper:

Voluntary corn oil ingestion increases energy expenditure and interscapular UCP1 expression through the sympathetic nerve in C57BL/6 mice

So of course the mice became obese. Okay, I'm making that bit up. Massive consumption of Rewarding corn oil does not make mice obese. They eat a little extra, admittedly, but that is because the metabolic effect of an 84% corn oil diet is to instigate uncoupling. A quirk of the biochemistry of linoleic acid, uncoupling proteins and rodent brown adipose tissue means that a certain amount of energy is wasted as heat, so the mice have to consume a little more corn oil to make up for this loss. A few extra total calories were consumed without any extra weight gain.

Metabolism + extra heat production = an extra amount of food has to be eaten to maintain normal weight.

Simple.

Going on to look at mechanisms of Reward, this study 

Increased Levels of Extracellular Dopamine in the Nucleus Accumbens and Amygdala of Rats by Ingesting a Low Concentration of a Long-Chain Fatty Acid

found that the Reward of corn oil is neuronal, it comes from local metabolism of the oil on the tongue using a lingual lipase so that a tiny concentration of FFAs is sensed by receptors in the mouth and nerve signalling from here drives the dopamine release in the brain. You can bypass the lipase on the tongue by adding as little as 1% unesterified linoleic acid to mineral oil which mimics the approximate amount of FFAs provided as a stimulus from the more normal corn oil/lipase process.

To make it absolutely clear:

Mineral oil carrying 1% of free linoliec acid is as Rewarding as 100% neat corn oil because both provide roughly the same FFA drive to the sensory cells. You can measure the dopamine release in the rodent brain. It's the same because the sensory cells on the tongue see the same concentration of FFAs.

I have a thought experiment.

What if one group of mice/rats were given chow plus access to neat corn oil and another group of mice were given chow plus access to mineral oil with 1% free linoleic acid added?

The oils are equally Rewarding.

Would the mice consume the oils in equal amounts because both are equally Rewarding?

If the oils were consumed in equal amounts would the mineral oil mice lose weight?

Or would they eat extra chow?

My bet is that if you ran such a study for eight weeks the mice with access to the mineral oil wouldn't touch it with a barge pole. It might release dopamine in the nucleus accumbens but it's not going to do much to fuel metabolism. That would need chow. Maybe there might be some recreational flavoured mineral oil consumption but I think that would pale quite rapidly.

Oooooooh! They might develop Reward Resistance! Like leptin resistance but dumber.

Reward is the refuge of researchers who consider obesity is caused by eating in excess of your metabolic needs.

A more sensible view of obesity is that it results from a metabolically mediated loss of calories in to adipocytes which then requires more calories to be eaten to meet normal metabolic needs. In the same way as orlistat requires over eating to compensate for fat loss via faeces, so "fake" corn oil (1% linoleic acid in mineral oil) would require extra chow to make up for the lack of calories in that highly Rewarding fake oil. And why feeding D12492 means rodents have to consume extra D12492 because they lose calories in their adipocytes.


Just to summarise: high Reward food makes you fat only if its metabolism result in energy sequestration in to adipocytes. 

Real corn oil at 84% of calories is not obesogenic because it does't sequester calories in to fat cells. I don't care how Rewarding it is. Nor does metabolism care.

Peter